The present invention relates to a piston for internal combustion engines.
A conventional piston for internal combustion engines is disclosed, for example, in Japanese Utility Model Application No. 55-9871. This piston comprises a ring land and a skirt connected thereto. The ring land is formed with ring grooves. The skirt is formed with a piston-pin hole with a center line which is offset to the counterthrust side of the piston with respect to a center line thereof. Struts having different weights are cast in the skirt in the inner wall on the thrust and counterthrust sides of the piston. The struts serve to restrain thermal expansion of an upper end portion of the skirt during engine operation, and to position a center of gravity of the piston nearly just above the center of the piston-pin hole.
The piston is arranged in a cylinder. A connecting rod is connected to the piston by a piston pin arranged through the piston-pin hole. During engine operation, the piston undergoes, before the top dead center of a compression stroke, a force in the counterthrust direction due to a pressing force out of the connecting rod inclined and a pressure of compressed gas, sliding on a counterthrust-side inner peripheral surface of the cylinder. When approaching the top dead center, the piston undergoes not only reduced force in the counterthrust direction since the inclination of the connection rod is decreased, but a counterclockwise moment since the piston-pin hole is offset to the counterthrust side of the piston.
As a result, the piston has a thrust-side upper end contacting a thrust-side inner peripheral surface of the cylinder, and a counterthrust-side lower end contacting the counterthrust-side inner peripheral surface of the cylinder. After this, the piston proceeds to an expansion stroke to undergo a force in the thrust direction due to a pressure of combustion gas and a reaction force out of the connecting rod. With increasing force in the thrust direction, the piston is moved downward with a portion between the thrust-side upper end and a lower end being widely in slide contact with the thrust-side inner peripheral surface of the cylinder.
In such a way, due to the piston-pin hole offset to the counterthrust side of the piston, the piston undergoes the counterclockwise moment in the expansion stroke. This moment operates to enlarge a clearance between an end of the skirt in the slide direction and the inner peripheral surface of the cylinder, enabling supply of lubricating oil to all thrust-side side surface of the piston.
However, the effect of restraining thermal expansion of the upper end portion of the skirt by the struts has a limit, and existence of the struts prevents the upper end portion of the skirt from being deformed, so that the skirt has in the vertical direction a great variation in the deformability which originates substantially from the vicinity of the oil ring groove. As a result, in order to prevent seizure, a thrust-side upper end of the skirt which contacts the inner peripheral surface of the cylinder in the expansion stroke needs to provide a taper with greater inclination in an upper portion of the piston.
This taper causes greater oscillating motion of the piston in the low-speed expansion stroke when the thrust-side upper end of the skirt contacts the inner peripheral surface of the cylinder. That is, the piston has kinetic energy increased when coming in slide contact with the inner peripheral surface of the cylinder, vibrating the cylinder to produce great hammering. Moreover, enlargement of the clearance between the upper end of the skirt and the inner peripheral surface of the cylinder results in small pressure acting area of the skirt. This increases a surface pressure on a thrust-side portion of the skirt, producing difficult formation of an oil film on a thrust-side peripheral surface of the skirt, resulting in possible increase in the friction thereat.
It is, therefore, an object of the present invention to provide a piston for internal combustion engines which has less hammering of the piston, and reduced friction between the piston and the cylinder in the expansion stroke.